The invention relates generally to mixers for fluids and specifically an apparatus for mixing a coagulant such as rennet and precheese.
The prior art has used both batch mixing and conduits with interior vanes/baffles to effect mixing. In the preparation of admixtures of coagulant and precheese, such procedures, using conventional mixing devices, often produce uneven mixing and, as a consequence, irregular coagulation occurs.
The mixer of the present invention is designed to effect rapid uniform mixture of two fluids and is most useful where its attributes are essential, that is, where the combination of fluids interacts or reacts and the product is non-uniform or otherwise detrimentally affected where production of a uniform mixture occurs over time. The advantage of rapid and uniform mixture, in accordance with the present invention, is magnified where (1) at least one of the fluids is a liquid of high viscosity and (2) the viscous liquid is mixed with a second fluid, which is present in small amounts and which, when combined with the viscous fluid, interacts or reacts with the viscous fluid to form a product that impedes or prevents further mixing and/or the uniform interaction and/or reaction of the fluids.
The mixer of the present invention, therefore, finds particular utility in the production of precheese and coagulant mixtures. Precheese is, at present, produced mostly by ultrafiltration of milk to produce a viscous retentate (ultrafiltered milk). The retentate, whose composition corresponds to the targeted cheese product, is thereafter mixed with a cheese-forming agent, a coagulant, such as rennet, to produce a cheese product.
The precheese, which is mixed with coagulant to form the feed that is treated in accordance with the present invention, is suitably formed by ultrafiltration using ultrafiltration devices described in my copending application, Ser. No. 08/694,125, now U.S. Pat. No. 5,817,235, entitled ANTI-TELESCOPING DEVICE FOR SPIRAL WOUND MEMBRANE MODULES, the teaching of which is incorporated herein by reference thereto.
The mixture of precheese and coagulant prepared using the mixing method and mixing device of the present invention can be treated to form a ribbon of cheese in accordance with the continuous cheese coagulation method described in my copending patent application, Ser. No. 08,694,228, now U.S. Pat. No. 5,688,542, entitled CONTINUOUS CHEESE COAGULATION METHOD, the teaching of which is incorporated herein by reference thereto.
A critical step in the production of admixtures of coagulated precheese is the mixing of rennet with the viscous milk retentate. Conventional mixing devices often produce uneven mixing and, as a consequence, irregular coagulation occurs. Irregular coagulation produces layers in the final cheese with poor, unacceptable structure. The viscosity of the retentate makes it difficult to evenly distribute the rennet, particularly because a very small amount of rennet is added to the retentate. The problem of irregular coagulation is exacerbated because retentate coagulates very rapidly in the presence of rennet. The resulting coagulant (solid) makes it very difficult, if not impossible, to disperse the remaining rennet within the remaining liquid retentate. The mixer of the present invention specifically addresses this problem. The sequence of events involved in the mixing of retentate and rennet, in accordance with the present invention, is altered from the conventional sequence, dosing, followed by injecting, followed by mixing (dispersion or diffusion). In the present invention, a new sequence has been adopted, namely, dosing, followed by xe2x80x9cdispersionxe2x80x9d of retentate (into a thin, annular film) followed by injection of rennet into the xe2x80x9cdispersedxe2x80x9d retentate.
In the prior art, mixing of rennet with retentate starts by dosing or measuring the relative amounts of the various components to be mixed. Then the retentate and rennet are introduced into a container for batch mixing or are kept flowing in a tube for continuous mixing. Various devices have been used to provide rotating or oscillating blades of various shapes and forms, generally activated by an external source of energy to promote the dispersion of rennet and retentate. For continuous in-line mixing, the liquid mix flows inside a tube with static mixing elements therein.
In the embodiment of the present invention relating to formation of precheese, the method of the present invention involves (a) forming a continuously flowing annular thin film of retentate; and (b) injecting rennet in predetermined small dosage levels into the thin film of flowing retentate. The angle of injection relative to the annular retentate film illustrated herein is radial to the axis of the annulus forming walls, however, any other angle relative to the surface of retentate is acceptable.
Also, in accordance with the present invention, further mixing, in addition to mixing caused by rennet injection into the moving retentate film may occur downstream of the spray/injection apertures.
In accordance with the present invention, dispersion, after combination of coagulant and precheese, is minimized by first dispersing retentate (into a thin annular film) before injecting rennet. In other words, the objective of mixing in the present invention, is to xe2x80x9cdispersexe2x80x9d the precheese and inject the coagulant so that first contact approximates as much as possible the architecture of a uniform dispersion of coagulant in retentate. Accordingly, the time consumed for combining components and the retention time after injection is minimized during the mixing continuum. The short duration of mixing, in accordance with the present invention, avoids partial coagulation. Accordingly, in the present invention, the components to be mixed are first divided into very small, easily-mixable portions. In the embodiment of the present invention relating to the production of precheese, this is done by creating a continuously-flowing thin-film of retentate and a synchronized stream of rennet Referring to FIG. 1, retentate film fills and flows through the annular space between the mixer body 230 and the mixer head 250. The rennet stream is injected, circumferentially into the flowing thin-film of retentate through a series of extremely fine nozzles formed at the interface of the mixer head and mixer body. Optionally, further mixing and/or finishing may occur downstream of rennet injection and as specifically shown in the illustrated embodiment of the mixer of the present invention, this can be achieved by frusto-conically shaped turbulence promoters placed downstream of the injection nozzles.
In the present invention, in order to minimize retention time within the mixer, the retentate is passed through the mixer at a high linear speed which provides the additional benefit of reducing the actual viscosity of the retentate. Most preferably the physical size (internal volume) of the mixer is kept to the absolute minimum required. Also contemplated in accordance with the present invention is the use of a finisherxe2x80x94an optional element that can be installed to effect further mixing. The finisher reinforces the dispersion of the rennet within the retentate and slows down the mix at the mixer outlet to avoid splashes in downstream unit(s) where the coagulant and precheese mixture formed by the mixer is coagulated and cheese is formed. The finisher of the present invention illustrated in FIGS. 8 and 9 results in only minimal addition to the internal volume of the mixer.